1. Field of the Invention
The present invention relates to a process for producing a base connection of a bipolar transistor having active regions of a first conductivity type, active regions of a second conductivity type, and low-resistivity regions of semiconductor/metal mixed crystals that connect to an inner subregion of the active regions of the second conductivity type in an electrically conductive manner.
2. Description of the Background Art
A base of a bipolar transistor manufactured with a planar process can be conceptually divided into an inner base and an outer base. The inner base forms flat, parallel pn junctions with adjacent emitter and collector layers, while the outer base provides electrical connection between the inner base and external contacts. For many applications of bipolar transistors, such as high frequency power amplification, for example, a base resistance is an important parameter, which limits the electrical characteristics of the transistor. The base resistance is frequently dominated by the resistance of the outer base.
In order to reduce the resistance of the outer base of bipolar transistors that are based on silicon technology, the outer base is frequently silicidized. In this process, a layer of a metal that reacts with silicon is deposited on the outer base and made to react with the silicon material of the outer base. By annealing above a transition temperature, the resulting suicides are converted into a modification having a low resistivity.
For many applications, specifically in the high frequency range, it is desirable to introduce an additional semiconductor material, for example germanium or carbon, into the inner base of a bipolar transistor, in addition to silicon. During epitaxial deposition of the semiconductor material for the inner base, the additional semiconductor material is, conventionally, also introduced along with silicon into parts of the outer base. Germanium and carbon have similar chemical properties to silicon, so that a metal that reacts with silicon also reacts with germanium or carbon under the same reaction conditions.
However, compounds of the metal and germanium exhibit poor thermal stability and dissociate at the silicide transition temperature. The germanium and metal deposits formed by dissociation thus exhibit a resistance-increasing effect.
Accordingly, when an additional semiconductor material is present in the inner base, it is desirable to conduct the silicidizing reaction such that the silicidizing front does not penetrate to the depth of the inner base. Conversely, a high silicide thickness is desirable for reducing the base resistance.
To avoid the aforementioned dilemma, where silicidization of a base connection produces decay products that increase resistance, until now a first process and a second process have been known, which are first described briefly and are later explained in detail with reference to FIGS. 1 and 2.
The first process for achieving a high silicide thickness in the outer base of bipolar transistors having silicon and germanium in the inner base while simultaneously stopping the silicidizing front above the layer containing the germanium is known in the conventional art. In the first process, a p-doped layer of a mixed silicon-germanium material and a thick n-doped silicon layer are grown on a collector region of n-doped silicon.
Next, a sacrificial dielectric structure is deposited on the thick silicon layer. Portions of the thick silicon layer are subsequently redoped by implantation into the p-doped outer base, with the sacrificial structure serving as a hard mask. Then titanium is deposited and caused to react with the material of the thick silicon layer. Where the sacrificial structure covers the thick silicon layer during titanium deposition, no silicide formation takes place. After removal of the remaining titanium, an additional dielectric layer is deposited and is exposed through CMP (CMP=chemical mechanical polishing) using the sacrificial structure as a polish stop. The hollow form remaining after removal of the sacrificial structure defines an emitter window that is self-aligning to the outer base and silicide, in which n-doped silicon material of the thick silicon layer is exposed, and that represents the emitter region of the transistor.
The thick silicon layer makes available silicon material for a thick silicide layer without having the silicidizing front penetrate to the depth of the layer of mixed silicon-germanium material. However, the thickness of the silicon layer also produces a large-area diode between the outer base and the emitter region of the transistor so that the resulting component, in addition to having a junction transistor including the emitter region, inner base, and collector region, also has a large-area surface-barrier transistor including the emitter region, outer base, and collector region. While the junction transistor can utilize the advantages resulting from the alloying of germanium with the silicon material of the inner base, this is not the case for the surface-barrier transistor, so the large-area surface-barrier transistor adversely affects the electrical properties of the resulting component.
The second process for achieving a high silicide thickness in the outer base of bipolar transistors having silicon and germanium in the inner base while simultaneously stopping the silicidizing front above the layer containing the germanium is known from the document “IEEE IEDM 2003, Technical Digest, Article 5.3.1,” hereinafter referred to as D2.
In the process according to D2, the silicon material required for achieving a high silicide thickness is deposited exclusively above the outer base in through selective epitaxy after completion of the emitter connection, and is subsequently silicidized. The process from D2 makes it possible to significantly reduce the area of the surface-barrier transistor as compared to the process from D1. However, the requirement in the process from D2 for selective epitaxy, a complex and expensive epitaxy process, is a disadvantage. Further disadvantages are described below in connection with the explanation of FIG. 2.